Closed pond system for wet process phosphate plants

ABSTRACT

CLOSED POND SYSTEM OR PROCESS FOR ELIMINATING THE CONVENTIONAL POUND SYSTEMS OF WET PROCESS PHOSPHORIC ACID COMPLEXES, AND TO REMOVE FLUORINE FROM PROCESS GAS STREAMS, WHEREIN LIQUID EFFLUENTS FROM WET PROCESS PHOSPHORIC ACID COMPLEXES, INCLUDING SCRUBBER LIQUOR FROM GAS SCRUBBING OPERATIONS OF THE COMPLEX, ARE COOLED AND CLARIFIED, AND THE LIQUID FROM THE CLARIFIER RECYCLED TO THE PHOSPHATE COMPLEX, THE SLUDGE OR SLURRY BEING FILTERED TO REMOVE SOLIDS, THE SOLIDS BEING CALCINED TO DRIVE OFF FLUORINE-CONTAINING GASES. THE CALCINED SOLIDS CONSIST PRINCIPALLY OF SODIUM FLUORIDE (NAF) WHICH IS RECYCLED TO THE CLARIFIER OVERFLOW TO BE MIXED WITH THE LIQUID RETURNED TO THE PHOSPHATE COMPLEX. IN THE PHOSPHATE COMPLEX THE LIQUID FROM THE CLARIFIER IS USED TO SCRUB PLANT GASES, AND IS MIXED WITH OTHER SCRUBBER LIQUORS FROM THE PLANT, AND RECYCLED AGAIN TO THE COOLING TOWER AND CLARIFIER. THE HF IN THE EFFLUENT PLANT LIQUIDS IS CONVERTED TO SIF4 BY MAINTAINING AN EXCESS OF SIO3, AND THE NAF REACTS WITH THE SIF4 TO FORM NA2SIF6. THE NA2SIF6 IS CALCINED TO PRODUCE NAF AND GASEOUS SIF4, THE LATTER BEING SCRUBBED WITH WATER TO FORM H2SIF6 OF COMMERCIAL QUALITY.

CLOSED POND SYSTEM FOR WET PROCESS PHOSPHATE PLANTS Filed Aug. 5. 1970 Match 13, 1973 R. s. HARTIG 2 Sheets-Swat 1 Rafa; 62 0 01 flaw by INVENTOR.

Afro/war MM 13. ms

R. G, HARTIG CLOSED POND SYSTEM FOR WET PROCESS PHOSPHATE PLANTS Filed Aug. 5. 1970 2 smu -5mm 2 United States Patent O 2 Claims ABSTRACT OF THE DISCLOSURE Closed pond system or process for eliminating the conventional pound systems of wet process phosphoric acid complexes, and to remove fluorine from process gas streams, wherein liquid eflluents from wet process phosphoric acid complexes, including scrubber liquor from gas scrubbing operations of the complex, are cooled and clarified, and the liquid from the clarifier recycled to the phosphate complex, the sludge or slurry being filtered to remove solids, the solids being calcined to drive off fluorine-containing gases. The calcined solids consist principally of sodium fluoride (NaF) which is recycled to the clarifier overflow to be mixed with the liquid returned to the phosphate complex. In the phosphate complex the liquid from the clarifier is used to scrub plant gases, and is mixed with other scrubber liquors from the plant, and recycled again to the cooling tower and clarifier. The HF in the eflluent plant liquids is converted to SiF, by maintaining an excess of SiO, and the NaF reacts with the SiF to form Na SiF The Na SiF is calcined to produce NaF and gaseous SiF the latter being scrubbed with water to form H SiF of commercial quality.

BACKGROUND OF THE INVENTION This process is designed to eliminate all contamination by gaseous fluorine liberated in phosphate rock processing from the waters used to slurry gypsum, in wet process phosphoric acid complexes, and to remove fluorine from process gas streams. In addition, various other benefits are obtained including fluorine recovery as a saleable product, elimination of liming costs, higher P 0 recovery, and the like.

In current practice, by-product gypsum from wet process phosphoric acid complexes is slurried with water and pumped to a pond of vast acreage in which the gypsum settles out and the supernatant water is cooled in ponds and recycled to process for use as scrubbing water to remove fluorine from process gas streams, as condensing streams in barometric condensers, and to slurry gypsum. The fluorine content of this water builds up to appreciable concentrations presenting undesirable problems, such as surface water and ground water contamination by fluorine. Addition of lime to these waters is frequently resorted to in order to control pollution of surface waters into which the plant waters may become intermingled.

By the novel process herein presented, it is possible to eliminate this type of fluorine contamination by completely eliminating the use of gypsum slurry water in scrubbing and processing. All fluorine contaminated scrubbing liquor is handled and stored in impervious, lined equipment separate from the gypsum slurry system.

The liquid solution circulated through this process and the complex has a pH of 4.5-4.7, and therefore has no fluorine vapor pressure because the fluorine therein is present as the two stable salts, Na SiF and NaF. This condition precludes the presence of any fluorine in the air exits of cooling towers, and in other equipment. The total gaseous fluorine emission from the entire closed pond system will be less than 15 pounds per day, for a 3,720,757 Patented Mar. 13, 1973 phosphate complex of the size wherein the volume of liquid eflluent is in the 16,000 gallons per minute range.

Other objects and advantages of the process herein disclosed will appear from the following detailed description of a preferred embodiment, reference being made to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1A and 1B are flow diagrams which together show a preferred embodiment of the process.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIG. 1, the eflluent leaving the phosphate complex enters the process at flowline 10, which leads into catch basin 11. The liquid contains approximately 0.66 percent fluorine, and is at a temperature of between F. and F. The fluorine is present as Na SiF which has been formed by the reaction between NaF and SiF A slight excess of reactive SiO is maintained in the recirculating liquid stream to convert any HP to SiF, (4HF+SiO SiF -|-2H O). The liquid also contains a small amount of NaF in solution.

The liquid is pumped from catch basin 11 by cooling tower lift pump 12, the flow being through flowline 13 containing valve 14 and, upon leaving the pump, through flowline 17 which delivers the liquid to the top of cooling tower 18. i

The liquid passes downwardly through cooling tower 18, while air is drawn in through plural side inlets 19 to pass upwardly through mist eliminator 20, the air being drawn upwardly by a blower 21 driven by a motor, not shown. The cooling tower may be packed or equipped with baffles, or the like, in any suitable manner. Any suitable cooling tower may be substituted.

The cooled liquor which leaves the cooling tower by gravity through flowline 23 is at a temperature of 90 F., maximum. The liquid is delivered through flowline 23 into the inlet 34 of clarifier 25. The clarifier may be of any suitable form. Clarifier 25 as shown includes paddles or drags 26 which rotate in order to collect the thickened slurry at the lower center of the clarifier. The clarifier has a downwardly converging bottom or floor, and has there-around an overflow trough 27 in flow communication with an overflow sump 28. Liquid overflow from the clarifier exits through flowline 29 containing valve 30, pumped by pump 31 through flowline 32 which delivers the overflow liquid from the clarifier back to the phosphate complex, wherein the liquor is used for scrubbing and/or other plant purposes, the liquid being circulated through the complex to be returned to the herein described process through flowline 10.

Fresh water is added as necessary to the overflow sump 28 through flowline 37.

The overflow liquor from the clarifier contains approximately 0.6% fluorine, present as Na SiF and a small excess of NaF in solution. This concentration of fluorine is approximately 0.06% less than the concentration of fluorine in the liquid entering the process through flowline 10. The excess fluorine in flowline 10 is derived from the phosphate complex scrubbing and washing operations.

The slurry leaving clarifier 25 at the center bottom thereof exits through flowline 39 having valve 40', being moved by pump 41 delivering the slurry through flowline 42 having valve 43 to settler 45. Settler 45 has a downwardly converging conical bottom, and is equipped with a motor driven paddle or drag 46. The overflow liquid from settler 45 is returned through flowline 47 to be mixed-with the cooling tower liquid output in flowline 23.

Fresh water is added to the slurry from clarifier 25 delivered through fiowline 42 to settler 45 through a fiowline 48.

The concentrated slurry discharged from the center bottom of settler 45 is delivered through fiowline 49 having valve 50, compelled by pump 51 through flowline 52 including valve '53 to a filter 54. The filter 54 is indicated in the drawing as being a rotary drum filter, but other suitable filtration apparatus may be suitably used. The filtrate from filter 54 is discharged through fiowline '55 by gravity to be mixed with the liquid overflow from clarifier 25 at overflow sump 2 8.

A water slurry of Si may be introduced to the clarifier overflow sump 28 through a fiowline 53 in order to maintain the SiO concentration in the liquid recirculated to the phosphate complex.

The solid materials collected at filter 54 are delivered by conveyor '58 to the feed 59' of a calciner 60. The calciner 60 is indicated in the drawing as being of the rotary drum, countercurrent flow type, but other suitable apparatus for this purpose may be substituted. Fuel and air are burned at burner 62 of calciner 60, the flue gas passing toward the left through the calciner to be discharged through flowline 63.

In calciner 60, Na SiF is decomposed at a temperature of 1350 F. to produce solid NaF and gaseous SiF The gaseous SiF is discharged from calciner 60 through fiowline 63 with the flue gases from the burner. A portion of the liquid in fiowline 32 is diverted through fiowline 65 to be introduced into NaF slurry tank 66, the amount introduced being controlled by valve 67. Slurry from tank 66 is circulated through fiowline 69 including valve 70 by pump 71 which delivers the slurry through fiowline 72 including valve 73 through the discharge 76 of calciner 60. Calcined NaF is discharged from the calciner to be mixed with the circulating slurry at 76. The slurry-solids mixture is passed through fiowline 77 to the slurry tank 66.

A portion of the slurry in fiowline 72 is passed through fiowline 79 to be delivered into the clarifier overflow sump 28. A portion of the slurry passing through fiowline 79 is circulated through fiowline 80 having valve 81 through the bottom of calciner feed 59, to be delivered through fiowline 82 for mixture with the slurry mixture in flowline 77. In this way, material at the calciner feed dust hood may be mixed with the slurry in slurry tank 66.

The flow through fiowline 79 delivers make-up NaF to the closed pond circulating liquid returned to the phosphate complex through fiowline 32.

The gases discharged from calciner 60 pass through fiowline 63, through scrubber 87, and therefrom through fiowline 88 to a second scrubber 89. After passing through scrubber 89, the gases are passed through fiowline 91 to a third scrubber 92-, and thence through fiowline 93 to a fourth scrubber 95. From scrubber 95 the gases pass through fiowline 96 to blower 97, and thence through rflowline 98 having valve 99 to stack 100.

Fresh water is introduced to scrubber 92 to wash the gases, through fiowline 103. The scrubber liquor from tower 92 exits through valve 104 and fiowline 105 through pump 106 which delivers part of the liquid in recirculation through line 107 having valve 108, and which delivers a part of the liquid through fiowline 109, flow through which is controlled by valve 110, into scrubber 89 wherein the scrubber liquor is contacted with the gases passing through the scrubber. The scrubber liquor is discharged from scrubber 89 through valve 114 and fiowline 115, and is recirculated to the top of scrubber 8 9 through fiowline 116, being propelled therethrough by pump 117. Flow through fiowline 116 is controlled by valve 118. Scrubber slurry is withdrawn from this system through fiowline 120 having valve 121.

Scrubber liquor from scrubber 87 is delivered through fiowline 125 to settler 126. Settler 126 has a downwardly converging conical bottom and has rotating paddles or drags 127. Slurry from settler 126 is discharged from the bottom center of the settler through valve 129 and fiowline 130 to be moved by pump 13-1 through fiowline 132 having valve 133. Liquid overflow from the settler is discharged through valve 135 and fiowline 136, pump 137 moving the liquid through fiowline 138 having valve 139 to be recirculated through the scrubber 87. Slurry from clarifier 25 in fiowline 42 can be diverted through valve 140 and fiowline 141 to the feed of settler 126. A portion of liquor from fiowline 65 is passed through valve 143 and fiowline 144 to scrubber 87.

Flowline 145 branching from fiowline 65 carries overflow liquid from clarifier 25 to the top of scrubber 95, fiowline 145 having a control valve 146. Recirculating liquor in tower 95 drains through valve 147 and fiowline 148 to be pumped by pump 149 through recirculation fiowline 150, fiowline 150 having valve 151. Scrubber liquor is withdrawn from fiowline 150 through valve 153 and fiowline 154 to be delivered into the feed inlet of clarifier 25. Liquid condensing in blower 97 is drawn through fiowline 156 back to scrubber 95. The scrubber has a mist eliminator 157 through which the exit gas passes into fiowline 96.

The gases leaving calciner 60 through fiowline 63 contain SiF derived from decomposition by heat of Na SiF according to the reaction,

(heat) NazSiFa 2NaF SiF4 In the described system, scrubber 87 is a recycling scrubber provided primarily to remove fluoride dusts prior to recovery of the fluorine, and to convert the NaH PO to H PO by the reaction The slurry from settler 126 delivered through flowline 132 is passed through a heat exchanger 161 (cooler). Liquid from fiowline 65 is passed through fiowline 162 to cool the slurry and is returned through fiowline 163 which communicates with fiowline 154. The cooled slurry is passed through fiowline 165 into agitated retention tank 166, from which it is delivered through valve 167 by pump 168 and through fiowline 169 having valve 170 into filter 172. Filter 172 is indicated as being a rotary drum filter, but other types of filtration equipment may be substituted. The filtrate from filter 172 is delivered through fiowline 174 to barometric seal tank 175, from which it passes through valve 176 and is pumped by pump 177 through fiowline 178 which delivers recovered phosphoric acid, as explained below, to the phosphate complex. The phosphoric acid solution may be delivered through valve 180 and fiowline 181 to settler 126. The cake from filter 172 is sluiced with liquor of any suitable nature and source or with water at 183 to be delivered through fiowline 184 to mix with the scrubber liquor returned to clarifier 25 through fiowline 154.

In the liquid circulation stream through cooling tower 18, clarifier 25, and the phosphate complex, phosphoric acid present in the liquor reacts with NaF and SiO,; to form Na-H PO which has a pH of approximately 4.7 which is in the pH range of the circulating liquor. The chemical reaction is;

6 The NaH PO concentration of the circulating liquor can be allowed to build up to 5 to 20 percent P 0 P 0 in the form of NaH PO is continuously removed from the circulating liquor which is delivered to scrubber 87. Gaseous SiF from calciner 60 reacts to form phosphoric acid as shown in the following chemical reactions:

The circulating liquor is controlled by checking the pH and if the pH is at 4.7 or above there is a suflicient excess of NaF and SiO: present. If the pH falls below 4.7 there is a deficiency of SiO, or NaF in the system. Insoluble Na SiF and SiO2, which enter the phosphoric acid recovery area through fiowline 132, are filtered out by filter 172 and recycled to the clarifier 25. The phosphoric acid obtained according to the above reaction is delivered through fiowline 178 to the phosphate complex, or used in other suitable manner.

Since the scrubber liquor of scrubber 87 is saturated with respect to SiF SiF (gas) continuously passes through fiowline 88 to scrubber 89, where the SiF; is recovered as a water solution of H SiF Any remaining SiE; is recovered in scrubber 92, and tail scrubber 95 picks up any residue so that the gases passing out of stack 100 are essentially fluorine free. The scrubber liquor passing through fiowline 120 is delivered through a filter 190, which is indicated to be of the rotary drum type, but for which any suitable type of filtration apparatus may be substituted. The filtrate passes through barometric leg fiowline 191 to seal tank 192. This filtrate is a water solution of H SIF which is delivered through valve 193, pump 194, and valve 195 through fiowline 196 to storage. This solution is a valuable commercial product.

The filter cake from filter 190, consisting of SiO, in pure form is delivered by conveyor 197 to slurry tank 198 to which Water is added through fiowline 199 concontrolled 'by valve 200. The SiO slurry is delivered through valve 203 by pump 204 through valve 205 and fiowline 206 to the plant, for addition of SiO, to the plant process where needed or for other use. A part of the SiO slurry is delivered through valve 208 and flowline 53, previously described, to the clarifier overflow sump 28.

Approximately 16,000 gallons per minute of plant effluent is delivered in a typical phosphate complex through fiowlines 10 and 17 to cooling tower 18, the liquid being of a specific gravity of approximately 1.1-1.2, at a temperature of approximately 109 F. The cooled eifluent in fiowline 23 is at the same flow rate and has the same gravity, and its temperature is approximately 90 F., maximum.

In flowline 32, approximately 16,000 gallons per minute of return liquor is returned to the phosphate complex, at a specific gravity of about 1.1-1.2 and a temperature of about 90 F., or less.

Scrubber 87 may be omitted from the system by neutralizing the solution in clarifier 25 with ammonia to form NH H PO NH H PO is water soluble, and is carried with the filtrate from filter 54 through flowline 55 to be returned to the plant. In this case, the system including filter 172 would, of course, be also eliminated.

Operating data are shown for a typical plant in the following Table I, providing an example of the process:

and separating the Na SiF from the circulating stream, the fluorine values are effectively removed from the phosphate complex effluent. By calcining the Na Si'F fluorine is recovered by water scrubbing as H SiF and NaF is recovered for re-use. Phosphate values are recovered as H PO or may be recycled to the plant as NH H PO The gases from the process are essentially free of fluorine, and no water solutions are disposed of to result in contaminations of surface and ground waters.

While a preferred embodiment of the invention has been shown in the drawings and described, many modifications may be made by a person skilled in the art without departing from the spirit of the invention, and it is intended to protect by Letters Patent all forms of the invention falling within the scope of the following claims.

I claim:

1. Process for removal of fluorine values from liquid eflluents from phosphate complexes, wherein the fluorine values are in the form, SiF and wherein the eflluents are returned to the phosphate complex for reuse after the fluorine values have been removed therefrom and are subsequently returned again to the process as liquid eflluents, comprising adding NaF to the liquid efiluent stream, said NaF disolving in the liquid efiluent stream and reacting to convert SiF, to Na Si'F separating Na SiF from the liquid effluent stream, calcining the Na Si'F to form NaF and SiF returning the NaF to the liquid eflluent stream, and scrubbing the Si F, with water to recover a solution of H SiF wherein the liquid eflluents from the phosphate complex also contain fluorine values in the form, HF, and comprising adding SiO to the liquid efiluent stream to convert the HF to Na SiF wherein the liquid eflluents from the phosphate complex also contain P 0 in the form, H P-O and wherein the H PO reacts with NaF and SiO to be converted to NaH PO which is separated from said Na SiF as a liquid eflluent stream, wherein said NaH PO is treated with a portion of said SiF, formed by said calcining step to convert the NaH PO to H PO and treating the remaining SiF formed by said calcining step with water to recover a solution of HgSiFg.

2. Process for removal of fluorine values from liquid eflluents from phosphate complexes, wherein the fluorine values are in the form, SiF and wherein the efliuents are returned to the phosphate complex for reuse after the fluorine values have been removed therefrom and are subsequently returned again to the process as liquid efiluents, comprising adding NaF to the liquid efiluent stream to convert SiF, to Na silf separating Na SiF from thel iquid eflluent stream, calcining the Na SiF to form NaF and SiF returning the NaF to the liquid eflluent stream, and scrubbing the SiF with Water to recover a solution of H Si'F the liquid efll-uents from the phosphate complex also containing fluorine values in the TABLE 1.--TYPICAL OPERATING CONDITIONS Flow rate Flow rate through Percent through Percent NaF SiF Flowfiowline Tempera- NaHaPOr NarSlF; Percent Percent NazSlFi Percent fiowline Percent H3P04 Lbs] LbsJ Percent line gaL/min. ture,F. (as P705) (as F) Nat S102 lbs./rnin. Hi0 s.c.t.m; 81F; (as P205) min. min. zSlFe 13, 000 106 0.5 SLXS SLXS 1 Trace. 2 Nil.

By conversion of the fluorine components (SiF, and HF) in the phosphate complex effluent to Na SiF by maintaining an excess of NaF and SiO in the liquid circulating through the process from the phosphate complex,

form, HF, and adding SiO to the liquid eflluent stream to convert the HF to Na Si-F the liquid eflluents from the phosphate complex also containing P 0 in the form, H -PO and the H PO reacting with NaF and SiO to be 7 converted to NaH PO which is separated from said Na SiF as a liquid efiiuent stream, said NaH- PO being treated with a portion of said Si=F formed by said calcining step to convert the NaI-I PO t0 H PO and treating the remaining SiF formed by said calcining step with water to recover a solution of H SiF References Cited UNITED STATES PATENTS 1,664,348 3/1928 Buchner 23-153 3,258,308 6/1966 Peterson et a1. 23- 153 2,954,275 9/1960 Carothers et a1. 23-165 OTHER REFERENCES Industrial and Engineering Chemistry, vol. 53, N0. 9 September 1961, p. 706.

OSCAR R. VERTIZ, Primary Examiner G. A. HELLER, Assistant Examiner US. Cl. X.R. 423311 a 

